Acrylonitrile is typically produced by reacting propylene, ammonia and air in the vapor phase over an ammoxidation catalyst. See for example U.S. Pat. Nos. 2,904,580 and 3,890,246. The vaporous reactor effluent from this reaction contains acrylonitrile, acetonitrile, water, HCN, side-reaction products and inert gases. The effluent is usually cooled to a temperature of 90.degree. F. to 230.degree. F. in a quench system to remove unreacted ammonia and various heavy polymers formed or in the reaction.
To recover the acrylonitrile, the prior art has cooled this stream to condense acrylonitrile and/or absorb the acrylonitrile in water in an absorber. The aqueous stream leaving the absorber contains acrylonitrile, acetonitrile, HCN and some impurities.
This aqueous solution then proceeds through various distillation steps to remove the acetonitrile and HCN. The steps can be found in U.S. Pat. No. 3,936,360.
After acetonitrile and HCN have been removed, the prior art then sends this aqueous solution to a drying column to remove water, and to a final product column wherein light and heavy impurities are removed such that specification acrylonitrile product can be obtained. This column operates under a vacuum to decrease polymer formation. Acrylonitrile is removed as a vaporous overhead stream or a liquid sidestream.
The present invention deals with an improved method for separating acrylonitrile from a solution containing acrylonitrile, water and impurities. The purification steps to obtain this solution from the gaseous reactor effluent are not critical, and vary widely. Common to these processing steps, however, is the need for a column to remove water and a final product column for purifying the acrylonitrile.